Since the late 1980s the Inductive Output Tube (also known as an “IOT” and a brand of which is marketed by Eimac under the trademark “Klystrode®”) has established itself as a useful device for broadcast, applied science and industrial applications in the UHF frequency range, typically operating in the 100 MHz–900 MHz range. Compared to a klystron, the IOT compensates for its lower gain with both superior efficiency and linearity, and it outperforms the tetrode, its next of kin in the electron device family, with regard to power capability and gain. However, it has long been thought that transit time effects limit the useful frequency range of IOTs to frequencies below 1000 MHz. It has been a commonly held belief in the industry that 1000 MHz is a hard threshold beyond which the performance of IOTs as fundamental frequency amplifiers would fall off rapidly.
FIG. 1 is a simplified electronic schematic diagram of a typical IOT 10 in accordance with the prior art. A cathode 12 held at a high negative potential compared to ground (typically a dispenser-type barium cathode) emits a beam of electrons 14. A control grid 16 fed by a radio frequency (RF) input source 32 density modulates the flow of the beam of electrons 14. An anode 18 held at ground potential accelerates the modulated electron beam 14. The modulated electron beam 14 passes through an output gap 20 where output power is extracted from the electron beam to an output resonator 19 by way of an induced electromagnetic field and directed to an output coupling 21 which is typically a coaxial feedline. A collector 22 receives the spent electrons. A grid bias supply 30 provides bias voltage to the grid, a beam power supply disposed between line 34 and line 38 provides the power to accelerate the electrons from the cathode to the anode, and a heater voltage supply 36 provides power to the heater of the cathode in a conventional manner. A solenoid magnet (not shown) typically surrounds the electron beam to focus it and reduce beam divergence. Input circuit 40 is shown schematically and acts to match the impedance of the input signal to the IOT 10.
The idea of employing higher-harmonic versions of IOTs at higher frequency bands was born early on. In a second-harmonic IOT, for example, the frequency-sensitive grid-cathode circuit (see, e.g., U.S. Pat. No. 5,767,625 entitled High Frequency Vacuum Tube with Closely Spaced Cathode and Non-Emissive Grid to Shrader et al.) could still be operated reliably in the well-experienced UHF regime, while the re-entrant output cavity could be tuned to a higher harmonic in an L-Band frequency. The main drawback to this approach is the relative length of the electron bunch that the low drive frequency forms. During its passage through the output gap the RF voltage in the output cavity changes its polarity twice: from the acceleration into the deceleration phase and back. Although the maximum of the current passes within the deceleration phase and thus ensures power conversion into the desired frequency, a considerable amount of electrons become accelerated, marginalizing efficiency and gain and causing problems with collector dissipation and X-ray radiation.
An investigation was conducted to see how far up in frequency the fundamental-frequency IOT could be tuned in computer simulation without jeopardizing its performance characteristics, particularly the operation of its critical grid-cathode configuration. An existing one-dimensional IOT computer code of proven reliability was modified to include the effects of grid-cathode transit time into the simulation.
As a first step an IOT electron gun with an established track record in UHF broadcast and science applications was analyzed to determine the change of electron bunch waveform and fundamental RF current versus frequency. The results of the simulation are shown in FIG. 3 which is a graph of simulated fundamental frequency current of an existing IOT gun versus frequency at 22 kV beam voltage and 47.4 V peak RF grid voltage operating in class B. Also interestingly, the useful fundamental RF current carried by the bunches in the simulation does not drop significantly until about 2 GHz (FIG. 3).
Accordingly, it would be highly desirable to develop a fundamental mode L-band IOT with reasonable performance characteristics.